Imported image darkened after imported to "image" node in compositing
I had the same issue. Check the box exposure value 

As you can see I had -2.610 value and the picture was darkened (like in your screenshots).

Try to set this value to 0. If the value changes back to previous number after a second, check your addons. As for me, I could set it to 0 only after disabling my Photographer addon